MSI installers now come in two versions.
In the past we had to build Rakudo on a x86 (32bit) platform to create an MSI installer package that is most portable. As of now we will also provide an x86_64 (64bit) installer package that additionally comes with JIT support.
If you have a 64bit system, probably Windows 7 or newer, the x86_64 version should give an extra performance boost compared to the 32bit version.
The two MSIs are available from http://rakudo.org/downloads/star/.